Once you call this method, any time the key's value is set, your observer
will be notified. Note that the observers are triggered any time the
value is set, regardless of whether it has actually changed. Your
observer should be prepared to handle that.

The sender is the object that changed. The key is the property that
changes. The value property is currently reserved and unused. The rev
is the last property revision of the object when it changed, which you can
use to detect if the key value has really changed or not.

Usually you will not need the value or revision parameters at
the end. In this case, it is common to write observer methods that take
only a sender and key value as parameters or, if you aren't interested in
any of these values, to write an observer that has no parameters at all.

When you call get on a computed property, the function will be
called and the return value will be returned instead of the function
itself.

Unknown Properties

Likewise, if you try to call get on a property whose value is
undefined, the unknownProperty() method will be called on the object.
If this method returns any value other than undefined, it will be returned
instead. This allows you to implement "virtual" properties that are
not defined upfront.

NOTE: If you do override init for a framework class like Ember.View,
be sure to call this._super(...arguments) in your
init declaration! If you don't, Ember may not have an opportunity to
do important setup work, and you'll see strange behavior in your
application.

Sometimes you need to change a value directly or indirectly without
actually calling get() or set() on it. In this case, you can use this
method instead. Calling this method will notify all observers that the
property has potentially changed value.

This method is generally very similar to calling object["key"] = value or
object.key = value, except that it provides support for computed
properties, the setUnknownProperty() method and property observers.

Computed Properties

If you try to set a value on a key that has a computed property handler
defined (see the get() method for an example), then set() will call
that method, passing both the value and key instead of simply changing
the value itself. This is useful for those times when you need to
implement a property that is composed of one or more member
properties.

Unknown Properties

If you try to set a value on a key that is undefined in the target
object, then the setUnknownProperty() handler will be called instead. This
gives you an opportunity to implement complex "virtual" properties that
are not predefined on the object. If setUnknownProperty() returns
undefined, then set() will simply set the value on the object.

Property Observers

In addition to changing the property, set() will also register a property
change with the object. Unless you have placed this call inside of a
beginPropertyChanges() and endPropertyChanges(), any "local" observers
(i.e. observer methods declared on the same object), will be called
immediately. Any "remote" observers (i.e. observer methods declared on
another object) will be placed in a queue and called at a later time in a
coalesced manner.